1) The effects of the inhibitors vanadate and oligomycin on the (Na,K)ATPase partial reactions were examined using the rapid mixing acid-quench technique. Vanadate produces complete inhibition of the enzyme by binding to the K ion-stabilized (E2) conformation and by activating the reversal of dephosphorylation. The latter effect may involve the interaction of vanadate with a neighboring subunit in an oligomeric enzyme complex. Investigations of the effects of monovalent cations on oligomycin inhibition showed that oligomycin interacts with (Na,K)ATPase in the absence of Na ion, but preincubation with Na ion potentiates its effect, suggesting that Na ion stabilizes the formation of an oligomycin binding conformation. 2) Studies of the interaction of ADP and Mg2 ion with the sarcoplasmic reticulum ATPase phosphoenzymes reveals the order of release of products from the enzyme during ATP hydrolysis. 3) Rapid ultrafiltration is being evaluated to achieve rapid separation of bound and free ligands in enzyme-metal ion complexation reactions.